He means "de-facto proprietary" in that there is no standard for the output of LaTeX, except whatever LaTeX outputs. That means anyone who wants to build anithe

He means "de-facto proprietary" in that there is no standard for the output of LaTeX, except whatever LaTeX outputs. That means anyone who wants to build anither version has a huge amount of work to do endlessly duplicating the quirks of the original implementation. Imagine HTML being defined as "whatever Firefox does". You're chasing a very complicated moving target and you'll always be behind if you aren't just copying the source wholesale. As to your second point, he mentions LaTeX converters in the article, saying you must write in a very restricted subset of LaTeX for it to convert properly. Obviously, pandoc doesn't have any way to turn everything LaTeX does into a markdown file. That being said, I personally like LaTeX a lot. But I wanted to clarify the points the author was making.